Abstract
ABSTRACTElasmobranch diets consist of high quantities of protein and lipids, but very low levels of carbohydrates including glucose. Reflecting this diet, most tissues use lipids and ketone bodies as their main metabolic fuel. However, the rectal gland has been shown to be dependent on glucose as a fuel, so we hypothesized that glucose transporters (GLUTs) would be present and upregulated in the gland during times of activation (e.g. following a meal). In this study, we searched for and identified putative class I GLUTs in three elasmobranchs and a holocephalan using transcriptomes, and used these to reconstruct a Bayesian phylogeny. We determined that each of the four species possessed three of the four class I GLUT sequences, but the identities of the isoforms present in each species differed between the elasmobranchs (GLUT1, 3 and 4) and the holocephalan (GLUT1, 2 and 3). We then used qPCR to measure mRNA levels of these GLUTs in the rectal gland, liver, intestine, and muscle of fed and starved spiny dogfish (Squalus suckleyi). The rectal gland data showed higher mRNA levels of GLUT4 in the starved relative to the fed fish. In the muscle, both GLUT1 and 4 were significantly elevated at 24 h post-feeding, as was the case for GLUT4 in the liver. In the intestine on the other hand, GLUT4 was significantly elevated by 6 h post-feeding, remaining elevated through 48 h. We suggest that GLUT4 has taken on the role of GLUT2 in elasmobranchs as the expression patterns observed in the liver and intestine are representative of GLUT2 in other vertebrates.
Highlights
Elasmobranchs are a primarily carnivorous group of vertebrates obtaining little glucose from their diet
We observed a significant increase in both GLUT1 and GLUT4 mRNA by 24 h post-feeding and this returned to fasted levels by 48 h (Fig. 6)
In this study, we sought to identify class I glucose transporters (GLUTs) from selected elasmobranch and holocephalan species as they have not been thoroughly investigated in cartilaginous fish, but are likely important transporters, in the rectal gland which requires glucose as a metabolic fuel
Summary
Elasmobranchs are a primarily carnivorous group of vertebrates obtaining little glucose from their diet. They have very low hepatic glycogen stores (0.5-2% compared to 6-8% in mammals and 2-6% in teleosts) due to the accumulation of lipids in the liver. These lipids are used for buoyancy and to produce ketone bodies, such as acetoacetate and β-hydroxybutyrate, which are the preferred fuel for many elasmobranch tissues making these species much less reliant on glucose than other vertebrates species (Ballantyne, 1997). A study by Balmaceda-Aguilera et al (2012) showed that the GLUT1 protein is expressed in the brain, and there has been speculation about the presence of GLUT2 and the absence of GLUT4 in elasmobranchs (Anderson et al, 2002)
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